20) $$\frac{3}{a} \cdot \frac{a^2}{12} \cdot \frac{8}{a^3} =$$

$$\frac{3}{a} \cdot \frac{a^2}{12} \cdot \frac{8}{a^3} = \frac{3 \cdot a^2 \cdot 8}{a \cdot 12 \cdot a^3} = \frac{24a^2}{12a^4} = \frac{2}{a^2}$$ Ответ: $$\frac{2}{a^2}$$